Surface activation by plasma jets for thermal spray coating on cylinder bores

ABSTRACT

A method of activating the surface of an aluminum-based substrate. This method includes cleaning the substrate surface, and operating a plasma spraying device such that a air plasma jet produced by the device is accelerated toward the surface so that it removes or decomposes any remaining oxides and other surface contaminants. In one form, the surface being treated forms part of a cylinder bore of an internal combustion engine block. In a particular form where a protective coating is subsequently being applied to the substrate, a sequential series of plasma spraying devices may be used such that a first—preferably simpler—device is used to perform activation or pretreatment operations, while a second—and more comprehensive—device may be used to deposit the protective layer on the pretreated surface.

BACKGROUND OF THE INVENTION

This invention is related generally to achieving better adhesion between a thermal sprayed protective coating and a target substrate, and in particular to pretreating the substrate with an air plasma spray prior to application of the protective coating.

Thermal spray techniques have been shown to be an effective way to deposit protective coatings—such as thermal barrier coatings, wear coatings, anti-corrosion coatings or the like—onto a workpiece. The high deposition rates make such coating approaches amenable to large-scale manufacturing, such as that associated with the production of engine cylinder bores and the pistons that are designed to reciprocate in them. Adhesion of the protective coating to a substrate is a very important metric for determining the suitability of the coating for a particular application (such as for the harsh environments produced within the combustion chamber of an internal combustion engine cylinder bore). Traditionally, improvements in coating adhesion to the cylinder bore substrate were achieved through various surface activation pretreatment steps, including approaches such as grit blasting with ceramic particles, high-pressure water jet blasting and mechanical roughening/locking. While effective for their intended purpose, they add significant complexity and cost to the coated component's manufacturing process. For example, mechanical roughening/locking-based approaches involve high tooling costs; these costs tend to be exacerbated by short tool life and extensive cleanup requirements. Likewise, the high-pressure water jet blasting approach has very high capital costs, while the grit blasting approach has sand contamination problems, as well as (along with the mechanical roughening mentioned above) significant cleanup requirements. Some of these cleanup requirements (as well as substrate pretreatment) may also use volatile organic compounds (VOCs) the use of which is coming under increasing scrutiny for their potentially negative environmental impact. Other attempts, including the deposition of a separate bonding coating, are also routinely used. As with the substrate activation pretreatments mentioned above, this involves significant additional complexity and related cost.

One particularly valuable form of thermal spray coating is through plasma spraying, where the constituents that make up numerous protective coatings are subjected an ionized flow of an inert gas. In addition to the high deposition rates, plasma spraying is beneficial in that the gas is chemically inert, while the target workpiece substrate may be kept relatively cool; these factors make it possible to avoid harm to both the impinging coating and substrate in a manner not possible with other elevated-temperature or chemically-active processes. In plasma spraying, oppositely-charged electrodes in the form of a cylindrical anode that circumscribes a bullet-shaped cathode form a flowpath that defines a nozzle at the discharge end. A direct current (DC) source is applied to the electrodes such that when the inert gas is introduced into an annular space between the electrodes, it is ionized to form a plasma that exits the nozzle as a jet stream. A separate coating insertion path (typically in the form of a tube) injects the protective coating material precursor (which is typically in powdered form) into the jet of plasma that develops in the nozzle. The device (typically referred to as a gun) operates when a current pulse is introduced between the anode and cathode such that it creates an arc through the gas and across the gap between these electrodes. The formation of the arc coincides with the electrons in the gas being stripped from their atoms and accelerated toward the anode, while the atoms are accelerated toward the cathode. A steady supply of electric current helps the arc to be pushed toward the exit in the nozzle, which in turn ionizes other atoms or molecules in the gas stream that results in a high-speed plasma that upon exiting the gun can be directed to impinge upon the suitable workpiece substrate.

An even more particular form of plasma spraying is known as plasma transferred wire arc (PTWA) thermal spraying. Unlike powder-based feedstocks, PTWA uses a sold wire that melts when subjected to the plasma jet produced by the gun. While plasma spraying in general (and PTWA in particular) has been especially well-suited to coating the aforementioned engine cylinder bores, it has not been used for substrate activation as a way to improve the ordinarily weak bonding exhibited between the protective coating and the substrate. Instead, recourse has traditionally been made to either the separate bonding coating or one or more of the substrate activation pretreatments mentioned above. There is a need for a simpler, less expensive approach that also reduces negative environmental externalities.

SUMMARY OF THE INVENTION

The current invention involves utilizing an air plasma jet produced from a plasma spraying apparatus to pretreat by activation the substrate before applying a protective coating. In this way, oxide layers may be removed, etched away or decomposed by melting or dissolving just prior to depositing the protective coating with this or another plasma spraying apparatus. In the present context, the term air plasma jet (also referred to herein as air plasma) differs from conventional plasma spray in that it is substantially devoid of any additives, such as a conventional protective coating material precursor. According to an aspect of the present invention, a method of activating the surface of an aluminum-based engine cylinder bore substrate includes cleaning the surface to remove at least a portion of an oxide formed thereon, and operating a plasma spraying device such that an air plasma jet produced thereby is accelerated toward the surface so that it decomposes or removes any remaining oxides and other contaminants that may otherwise impact the ability of the surface and a subsequently-applied protective coating to adhere to one another. In a preferred form, the substrate being exposed to the cleaning and the plasma jet is an engine cylinder bore. In another preferred form, the plasma spraying device is a pretreatment plasma spraying device that is separate from a plasma spraying device used to deposit the subsequent protective coating on the engine cylinder bore in such configuration, the pretreatment plasma spraying device is simplified. One significant way it is simplified is to be devoid of any protective coating material precursor receiving mechanism such that it may operate at a lower power setting than would otherwise be required, although in another form, it could be achieved by a traditional plasma spraying device where the protective coating material precursor receiving mechanism has been disabled; either variant is deemed to be within the scope of the present invention. In another preferred embodiment, the activation of the substrate is accomplished without being subjected to separate mechanical activation step (such as those discussed above in the Background section of this disclosure). In the present context, the terms “activation” and “pretreatment” are used interchangeably to describe the process of improving the surface characteristics of the target substrate with an air plasma jet so that long-term adherence of a subsequently-applied protective coating (such as low and high carbon steel wear-resistant coatings such as 0.1-0.8 wt % carbon, as well as the steels containing other alloying elements for corrosion and wear protection (such as Cr, Ni, Cu or the like) is improved. In another preferred form, the surface activation achieved by the present invention is done so without having to resort to traditional mechanical activation approaches such as those discussed above.

According to another aspect of the present invention, a method of coating the surface of an aluminum-based substrate is disclosed. The method includes cleaning the surface to remove at least a portion of an oxide formed thereon, operating a first plasma spraying device such that a plasma jet produced by the first device impinges on the cleaned surface, and then operating a second plasma spraying device such that a protective coating material precursor introduced into it impinges on the cleaned surface that has been pretreated with the plasma jet from the first plasma spraying device. In this way, their surfaces are being exposed to the first and second plasma spraying devices in sequential fashion.

According to yet another aspect of the present invention, a method of coating a cylinder bore of an engine block is disclosed. The method includes cleaning the surface with a solution containing at least one of potassium and fluorine, operating a first plasma spraying device such that a pretreatment air plasma jet produced by the first device impinges onto the cleaned surface, and operating a second plasma spraying device such that a protective coating material precursor introduced into the plasma jet produced in the second device impinges on the pretreated surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which the various components of the drawings are not necessarily illustrated to scale:

FIG. 1 depicts an isometric view of a notional engine block with four cylinder bores formed therein that could receive a protective coating according to an aspect of the present invention;

FIG. 2 depicts a plasma spray gun that may be used in conjucntion with the present invention; and

FIG. 3 depicts the cooperative placement of the plasma spray gun of FIG. 2 with the wall of an engine cylinder bore such that the gun may be used to pretreat the wall or depsoit a protective coating onto the wall.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, a simplified view of four-cylinder automotive internal combustion engine block 100 is shown. The block 100 includes portions for—among other things—the crankcase 110, the crankshaft bearing 120, the camshaft bearing 130 (in the case of engines with overhead valves and pushrods), water cooling jackets 140, flywheel housing 150 and cylinder bores 160. These bores 160 may include an alloyed surface layer (not shown) that is either integrally formed with the substrate of each bore 160, or as a separate insert or sleeve that is sized to fit securely within. In one form, such alloyed surface layer can be used to enhance the corrosion, wear or thermal resistance of the bore 160. In fact, in engine configurations where the block 100 is cast from a lightweight material, such as aluminum and its alloys (such as A380, A319 or A356), the addition of such surface layers was traditionally deemed to be necessary as a way to impart additional thermal and wear resistance. In one form, this alloyed surface layer is made from a heavy cast iron or related material.

Referring next to FIG. 2, the plasma spraying process involves the latent heat of ionized inert gas (plasma) being used to create the heat source. A plasma spraying device (also referred to herein as a plasma spray gun or more simply as a plasma gun) 300 that can be used as part of the present invention is shown in partial cutaway view. Gun 300 includes a housing 310 with a coolant channel 320 formed therein, as well as a plasma gas injection port 330 and protective coating material precursor injection port 340. As will be discussed in more detail below, in one preferred embodiment, when gun 300 is configured to act as a stand-alone device for surface activation/pretreatment operations using an air plasma jet rather than as a way to also deposit a protective coating, the protective coating material precursor injection port 340 may be removed or disabled. An external DC current source is connected to the bullet-shaped cathode 350, while the surrounding portion of the housing 310 forms the anode 360. In one form, cathode 350 is made from a thoriated tungsten, while the anode 360 is made from concentrically-shaped copper.

The most common gas used to create the plasma is argon; this is referred to as the primary gas and is made to flow between the electrodes and a nozzle 390. A high frequency or high voltage alternating electric arc is formed between the nozzle 390 and the anode 360 to ionize the gas stream. By increasing the arc current, the arc thickens and increases the degree of ionization. This has the effect of increasing the power and also, due to the expansion of gas, an increase in the velocity of the gas stream. If the gas being used as the arc plasma flame is substantially pure argon, a very large arc current is needed to create sufficient power to melt most of the protective spray coating precursor materials that are used in traditional plasma spraying operations. With this level of arc current, the gas velocity may be too high to melt many refractory materials. To overcome this, secondary gases (such as hydrogen) may be added to change the thermal and electrical properties of the gas stream such that the gun 300 power level is increased sufficiently to melt such refractory materials (even ceramics). Once the appropriate gas stream has been established for the material being sprayed, the feed stock (wire or powder spray material) for the material being deposited in a plasma spraying process is injected into the gas stream.

Air plasma can be generated with the same principle as described above, although the power needs are much lower. Thus, a gun (such as gun 300 shown and described herein) that is devoid of the need for the higher power requirements associated with a full protective coating material precursor injection capability may be used for surface activation through an air plasma jet as a separate air plasma spraying device. As such, it can be used as part of a more comprehensive protocol that has separate pretreatment and plasma thermal spray coating devices; such a configuration may be used to lower the total system capital cost (as well as operational cost). Thus, in one preferred embodiment, the gun 300 used as part of the present invention may be coupled to a production line-based manufacture of internal combustion engines in general and the walls or bores formed in cylinder blocks in particular. The gun 300 may be made to pivot about an axis that moves along the piston travel direction within the bore 160 so that it coats a substantial entirety of the bore 160 inner surface periphery while rotating circumferentially along cylinder wall. Upon application of the DC current and a switched pulse (to create arc 370), a relatively cohesive plasma jet 380 is ejected from a nozzle 390 in order to pretreat the desired surface. In configurations where gun 300 is capable of applying a plasma sprayed coating (such as that made with the precursor wire or powder the latter of which is introduced through protective coating material precursor injection port 340) as shown in FIG. 2, the plasma jet 380 includes the melted material droplets 385; otherwise, the plasma jet 380 is of the air plasma variant where no such droplets 385 are present.

In one form of the comprehensive protocol mentioned above, a first gun 300 (which does include a protective coating material precursor injection port 340) or plasma wire arc coating system (such as through PTWA) may be used; either are deemed to be compatible with the air plasma pretreatment of the present invention. In another form, gun 300 is configured to perform both the air plasma pretreatment and protective coating deposition operations; in this configuration, the protective coating material precursor through port 340 is present such that prior to the introduction of a protective coating material precursor therethrough, the plasma jet 380 is activated without the introduction of the protective coating material precursor to facilitate plasma bombardment of the target substrate with only the plasma jet 380 before the actual plasma coating process is initiated.

As will be discussed in more detail below, in one embodiment, gun 300 may be used for the deposition of the protective coating, while in a preferred embodiment, a simplified version of gun 300 that does not include a provision for introducing the protective coating material precursor therein may be used in cooperation with the more comprehensive gun 300 that does include the protective coating material precursor through port 340 such that the simplified version is used solely for the pretreatment operation while the more comprehensive version (such as shown in FIG. 2) is used exclusively for the coating deposition operation. In such configuration, the gun being used for pretreatment (also referred to herein as an air plasma gun in that it only dispenses an air plasma jet) constitutes a first plasma spraying device, while that used for protective coating deposition constitutes a second plasma spraying device.

Significantly, the plasma jet surface activation of the present invention avoids the difficulties associated with using organic and silicone-based materials while still promoting more full wettability of the substrate. By replacing traditional mechanical roughening (mentioned above) and VOC-based cleaning processes with inert plasma gun gases (for example, a mixture of argon and hydrogen), capital costs and subsequent environmental impacts are reduced. The plasma jet 380 gas temperatures and velocity distributions range widely depending on numerous factors, including nozzle 390 design, power levels and gas compositions. As mentioned above, the plasma gas is preferable an inert gas. One form of suitable plasma gas is argon, often with hydrogen or another secondary or auxiliary gas. Argon alone creates a relatively low-energy plasma related to its breakdown and thermal heat capacity, while other inert gases, such as nitrogen, produces a relatively hot plasma gas; use of one gas versus the other may be dictated by other factors, such as the propensity for reaction with other materials. Other additives, such as helium may be used to form a mixture (for example, an Ar/He mixture with approximately 20 to 50 percent helium by volume); this addition may help improve the thermal conductivity of plasma mixture, which in turn increases the plasma's heat capacity. In a similar way, argon/hydrogen mixtures (for example, approximately 5 to 15 percent hydrogen by volume) are also used and provided increased jet temperatures (i.e., enthalpy) over argon alone or argon/helium mixtures due to hydrogen's diatomic structure and its high collisional cross section related to its low mass.

As mentioned above, in one form, a gun 300 from an existing plasma spray coating system (which includes the protective coating material precursor injection port 340 or related material introduction apparatus) may be used, while in another, a separate (i.e., dedicated, or stand-alone) gun that requires lower power and only uses air as a plasma working fluid without a separate protective coating material precursor through port 340 may be used in conjunction with gun 300; an example of such a dedicated gun is commercially available from Plasmatreat North America of Elgin, Ill. One benefit of having a separate gun for the pretreatment and a separate gun for the protective coating deposition is that the high power densities (40-150 kW) being utilized by conventional DC-arc plasma guns create higher electrode erosion rates, which in turn may necessitate more system maintenance. Likewise, more thermal and electrical protection for gun components may be required. This in turn may necessitate a much more robust cooling system, where high pressure (for example, above 1 MPa) water cooling and high flow (3 to 12 liters/minute) are needed; because these require high-pressure seals and passages inside guns, the cost of not only the gun (where the copper nozzle has a relatively frequent replacement rate, with lives from 2 to 300 hours being reported and 10 hours being average) but also ancillary equipment may become prohibitively high if a single gun were to be used for both operations.

Once the cylinder bores 160 are bored to the desired size, their surface is cleaned by immersing in a 0.5M potassium-fluoride bath; this solution etches away the oxide layer that forms on the bore 160 surface, and then reacts with the now-exposed aluminum to form a K₃AlF₆ and KAlF₄ flux. The jet from the plasma gun 300 is made to impinge onto the fluxed surface to thermally activate the flux; this has the effect of melting the salt and dissolving any remaining surface oxides. In an alternate approach to the potassium-fluoride bath, the cylinder bore 160 is prepared by soak cleaning followed by an acid dip. In one form, the acid solution used for the dip is nitric acid (up to 50%); this dip may contain a small amount of fluoride either from hydrofluoric acid or a fluoride salt. Immersion of the substrate to such dip is preferably in the range of about 1 to 10 minutes, as excess etching can damage parts and cause pitting. Thorough rinsing is required between each of the exposures to the acid dip.

Referring next to FIG. 3, the use of gun 300 that can be used to pretreat the inner wall of an engine cylinder bore is shown. It can be used directly in line—conveyor belt as well as robotic application is possible. High speed up to 40 meters/minute of plasma jet treatment may be achieved. The method of the present invention is preferably used as part of a production line-based manufacture of internal combustion engines in general and the walls or bores formed in cylinder blocks 100 in particular. It can also be used for treating parts other than engines that require thermal spray coatings for good coating adhesion. By having the gun 300 mounted onto a rotating stem, it can be made to provide complete circumferential surface pretreatment that is defined by the wall or bore. As such, the approach of the present invention avoids the necessity of having the larger (and therefore more cumbersome) coated component be moved during the wall surface preparation and subsequent coating.

Cylinder bore 160 of engine block 100 is defines a circumferential inner wall 160A. As mentioned above, a stem in the form of a pressurized axial fluid conduit 200 may be used as a secure mounting platform for gun 300 (shown presently in simplified form). The stem may be made to rotate. Details of the cooperation between the rotating axial fluid conduit 200 and its use in cylinder bore 160 may be found in co-pending U.S. application Ser. No. 14/335,974 entitled NON-DESTRUCTIVE ADHESION TESTING OF COATING TO ENGINE CYLINDER BORE that is owned by the Assignee of the present invention and incorporated herein by reference in its entirety.

It is noted that terms like “preferably”, “generally” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.

For the purposes of describing and defining the present invention, it is noted that the terms “substantially” and “approximately” and their variants are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

Having described the invention in detail and by reference to specific embodiments, it will nonetheless be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. In particular it is contemplated that the scope of the present invention is not necessarily limited to stated preferred aspects and exemplified embodiments, but should be governed by the appended claims. 

What is claimed is:
 1. A method of activating the surface substrate of an aluminum-based engine cylinder bore to achieve better adhesion between a subsequently-applied protective coating and the substrate, said method comprising: cleaning said surface to remove at least a portion of an oxide formed thereon; and operating a plasma spraying device such that an air plasma jet produced thereby impinges on said surface.
 2. The method of claim 1, wherein said plasma spraying device defines a pretreatment plasma spraying device that is separate from a plasma spraying device used to deposit a protective coating on said surface.
 3. The method of claim 2, wherein said pretreatment plasma spraying device is configured such that it is incapable of receiving a protective coating material precursor therein.
 4. The method of claim 1, wherein said activating is accomplished without said surface being subjected to separate mechanical activation step.
 5. The method of claim 1, wherein said cleaning comprises exposing said surface to a potassium-fluoride solution such that at least one compound containing potassium, aluminum and fluorine is formed on said surface.
 6. The method of claim 5, wherein said at least one compound containing potassium, aluminum and fluorine comprises at least one of K₃AlF₆ and KAlF₄.
 7. The method of claim 1, wherein said cleaning comprises exposing said surface to an acid solution.
 8. The method of claim 7, wherein said acid solution comprises nitric acid.
 9. The method of claim 8, wherein said nitric acid solution further comprises at least one of hydrofluoric acid and a fluoride salt.
 10. The method of claim 9, further comprising rinsing said surface after each exposure to said nitric acid solution.
 11. The method of claim 1, further comprising depositing a protective coating material precursor onto said surface once said activating is substantially complete.
 12. The method of claim 11, wherein said depositing a protective coating material precursor onto said surface is performed by plasma spraying.
 13. A method of coating the surface of an aluminum-based substrate, said method comprising: cleaning said surface to remove at least a portion of an oxide formed thereon; operating a first plasma spraying device such that an air plasma jet produced thereby impinges on said cleaned surface; and operating a second plasma spraying device such that a protective coating material precursor introduced therein impinges on said cleaned surface that has been pretreated with said air plasma jet.
 14. The method of claim 13, wherein said surface defines an engine cylinder bore.
 15. The method of claim 14, wherein said first plasma spraying device is configured to operate to not introduce a protective coating material precursor therein.
 16. The method of claim 13, wherein said operating a first plasma spraying device takes place at a lower power setting than said operating a second plasma spraying device.
 17. A method of coating a cylinder bore of an engine block, said method comprising: cleaning said surface with a solution containing at least one of potassium and fluorine; operating a first plasma spraying device such that a pretreatment plasma jet produced thereby impinges on said cleaned surface; and operating a second plasma spraying device such that a protective coating material precursor introduced therein impinges on said pretreated surface.
 18. The method of claim 17, wherein said pretreatment plasma jet produced by said first plasma spraying device is an air plasma, and wherein said plasma coating produced by said second plasma spraying devices includes at least one of argon, hydrogen and helium. 